The invention disclosed herein relates generally to pressure diffusers for washing pulp and particularly relates to upper and lower cylindrical bearings between a moving screen assembly and stationary bearing cylinders in the pressure diffuser.
The term “pulp” generally refers to comminuted cellulosic material, such as wood chips that have been processed in a digester to separate the fibers in the wood. Chemicals, e.g., liquor, are injected into the digester vessel to process the pulp. After the pulp is discharged from the digester vessel, the pulp may have residual amounts of chemicals.
The pulp flow from the digester vessel to a pressurized diffuser that washes the pulp to remove the residual chemicals. A pressurized diffuser is typically a large vessel, e.g., 50 feet in height or greater. Pulp with chemicals enters an annular space inside the diffuser. Wash water is injected into the annular space and flows through the pulp to remove the chemicals from the pulp. The wash water with chemicals (referred to as “wash filtrate”) passes from the annular space through slots in an internal screen assembly. The slots allow the wash filtrate to pass through to an internal chamber at the center of the screen assembly. The slots in the screen assembly are too narrow to pass the fibers or other particles in the pulp. The cleaned pulp is typically discharged from the top of the pressurized diffuser. The wash filtrate is typically discharged from a bottom outlet in the pressure diffuser.
The screen assembly moves within the pressure diffuser. Traditionally, the screen assembly moves reciprocally up and down during operation of the pressure diffuser. The screen assembly may also rotate during operation. The movement of the screen assembly promotes the flow of pulp through the annulus in the diffuser. Particularly, the movement of the screen assembly assists in clearing the slots of fibers and particles that may be blocking the slots.
Cylindrical bearings support the screen assembly in the pressure diffuser. The bearings allow the screen assembly to move vertically with respect to the diffuser housing. The bearings are arranged at upper and lower regions of the diffuser. The bearings are sandwiched between the screen assembly and a stationary bearing cylinder in the pressure diffuser.
The cylindrical bearings are adjacent the upper and lower ends of the annular space containing the pulp. A difficulty with conventional cylindrical bearings has been that sand, fiber, rocks and other impurity particles in the pulp inadvertently enter the gap between the screen assembly and the bearing cylinder, and become caught against a surface of the cylindrical bearing. These impurity particles damage the surface of the bearing cylinder, such as by gouging the surface and forming grooves and other imperfections in the surface of the cylindrical bearing. The damaged surface of the bearing cylinder tends to allow fibers from the annular region with the pulp to move past the bearing cylinder and enter the filtrate chamber in the center of the screen assembly.
The damaged surface of the bearing cylinder can also lead to permanent failure of the bearings. Additionally, the difficulty with sand and impurity particles entering the annular space between the cylindrical bearings and the bearing surfaces is most pronounced when the cylindrical bearing is cool. When cool, conventional cylindrical bearings contract and open gaps between the bearing and the bearing cylinder.
Conventional bearings are formed of a soft plastic material, e.g., a polytetrafluoreoethylene (PTFE) or other fluorocarbon plastic such as Rulon™, that expands under heat. The thermal expansion of the conventional bearings creates a tight seal between the bearing surfaces, e.g., the screen assembly and stationary bearing, and the cylindrical bearing. The expansion coefficient of conventional cylindrical bearings has been about ten (10) times the expansion coefficient of the metal materials, e.g., stainless steel, used to form the bearing surfaces in the screen assembly and bearing. The pressure diffuser typically can operate at temperatures of up to 330 degrees Fahrenheit (F.) and about 150 degrees Celsius (C.). At these elevated temperatures, conventional cylindrical bearings have expanded to form tight seals in the gap between the screen assembly bearing surface and a bearing surface of a bearing in the pressure diffuser.
When the pressure diffuser has cooled, such as when taken off-line for maintenance and service, the temperature of the cylindrical bearings may drop to ambient temperatures, such as 32 degrees F. or to zero degrees C. At these cooler temperatures, the cylindrical bearings contract and open a gap between the bearings and the bearing surfaces on the screen assembly and bearing. The gap between a cooled cylindrical bearing and the bearing surfaces may be sufficient to allow sand and other impurity particles to enter and become trapped against the bearing when the pressure diffuser heats during operation.
When the cylindrical bearing is cool, sand, rocks and other impurity particles become trapped against and embed in the soft surfaces of the cylindrical bearing. The embedded sand, rocks and other particles scrape against the surface of the bearing cylinder as the screen assembly (with cylindrical bearing) moves reciprocally up and down, and may rotate. The scraping of sand, rock and other particles can damage the bearing cylinder.
The damage caused by sand and other impurity particles to cylindrical bearings in a pressure diffuser has created a long felt need for an improved cylindrical bearing. The damage to the bearings results in fibers entering the filtrate extracted from the pulp and impurities. Because of the damage caused by sand and other impurities, the cylindrical bearings are replaced periodically. The replacement of the bearings requires the pressure diffuser to be taken off-line and results in an interruption in the pulp cleaning process and, thus, loss of time and money due to repair and maintenance of the diffuser and lost pulp production.
Typically, cylindrically bearings are replaced every year or every year and an a half. However, the cylindrical bearings may require more frequent replacement, such as three or four times a year, if the damage to the bearing due to impurity particles causes an excessive amount of fibers to enter the filtrate. There is a need for improved cylindrical bearings that are less susceptible to the encroachment of sand and other impurity particles into the space occupied by the bearing. Preferably, the improved cylindrical bearings will have an operational life of at least one year, even when operating with pulp having relatively large amounts of fine sand or other small particulate impurities.